1. Field of the Invention
The present invention relates to the distribution and reception of compressed video information. More particularly, the invention relates to providing constant bit rate distribution of variable bit rate-encoded video programs while facilitating the distribution of auxiliary data of a general character to one or more receivers; and to receiving the distributed video information and creating a customized video program for presentation at each individual receiver.
2. Background
The present invention relates to the encoding, formatting, distribution and reception of compressed video programs. Video programs are often encoded using a particular video compression technique known as variable bit-rate (VBR) encoding. VBR encoding can be used to overcome the well-known problem of most video compression encoders wherein the image quality tends to vary as a function of image complexity. Typically, a video program will contain a variety of scenes. Many of these scenes are lacking in motion or detail and are therefore easily compressed. However, other scenes containing complex details may be more difficult to compress, particularly when moving in a complex or random manner. Therefore, unless the available bandwidth is very high, the perceived quality of the decompressed and reconstructed images will tend to vary from one scene to the next. This problem becomes more serious as the available bandwidth is reduced until, eventually, the video becomes unacceptable, often because of just a few problem scenes.
VBR encoding overcomes this problem by allocating more bits to those scenes which are difficult to compress and fewer bits to those scenes which are more easily compressed. In this way, the decompressed and reconstructed images can be made to appear consistently uniform and therefore superior to the reconstructed images derived from a constant bit-rate (CBR) encoder adjusted for the same average rate of compression. As a result, it is possible to compress a video program more efficiently by using the VBR encoding technique. This increases the number and variety of programs or program streams that can be delivered over a fixed-bandwidth communication channel, reduces the storage capacity requirements at the head end or other site where the program library is maintained, and reduces the storage capacity requirements at the head end or other site where the program streams are to be distributed over a fixed-bandwidth communication channel.
The disadvantage of the VBR encoding technique is that it presents certain problems when manipulating or editing the compressed bit-streams. In particular, it becomes difficult to efficiently utilize a fixed-bandwidth communication channel since the variable bit-rate stream may at times exceed the capacity of the channel, while at other times, it may utilize only a fraction of the available channel capacity. As would be clear to one in the art, the possibility of exceeding the capacity of the channel is also a problem with non-VBR encoded data.
One known technique that is used to alleviate this problem is to buffer the compressed bit-stream at the transmission end of the communication channel in order to convert the variable rate stream to a constant rate stream. In such a case, it is also necessary to buffer the signal received at the other end of the channel in order to recover the variable rate stream that is necessary for proper timing of the reconstructed video images. Unfortunately, the required amount of buffering required to convert a variable rate input to a constant rate output, would be prohibitively expensive and would introduce long delays into the distribution system. Moreover, existing video compression standards such as the MPEG standard specify limits on the amount of buffering required for conforming decoders. Therefore, it is important that the received bit-streams be decodable without exceeding these limits.
Another prior art technique that can be used to reduce the inefficiency of distributing VBR encoded (and non-VBR encoded) programs over a fixed-bandwidth channel combines a plurality of program streams into a single multiplex. Although each additional program stream will increase the overall data rate of the multiplex, the average per-stream variance compared to the variance of the original individual streams will tend to decrease in approximate proportion to the number of streams, assuming approximate statistical independence among them. Therefore, if the nominal rate that can be accommodated by the channel is significantly greater than the average rate of a single program stream, then the channel utilization can be significantly improved. This technique is known in the art as statistical multiplexing.
When using buffering, one prior art method of assuring that buffer overflow does not occur is to provide feedback between the buffer and the encoder. When the buffer approaches a full state, a buffer fullness signal from the buffer informs the encoder to reduce the bit-rate so that the buffer does not overflow. When the buffer has more room, the feedback signal, from the buffer to the encoder, enables the encoder to increase the bit-rate in order to maintain image quality. Such feedback is particularly effective when combined with statistical multiplexing. In such a case, a single buffer may be at the output of the multiplexer, and feedback from that buffer is to all the encoders of the programs being multiplexed. Because the multiplexing reduces effective per-stream data rate variance, in general, less buffering would be required to convert the multiplex to a fixed bit-rate stream than if each individual encoder included a VBR-to-constant bit-rate buffer.
The combination of buffering with statistical multiplexing with feedback for conveying encoded (particularly VBR encoded) program streams over fixed-bandwidth channels, can be effective but generally may not be sufficient when efficient utilization of the entire channel is important.
In addition, there are often situations where no feedback is possible between the output buffer of the multiplexer and the individual encoders. One such situation occurs when multiplexing previously encoded data streams. Another occurs when the encoders are located in an area physically remote from the multiplexer. Both these situations are referred to herein as remote encoding, indicating that encoding and multiplexing are remote in time, location, or otherwise, so that no feedback is possible from the multiplexer to the encoders of the program streams to be multiplexed.
In many applications a library of auxiliary data, which exists independently of the encoded video data, may need to be selectively distributed to the same set of receivers that receive video programs, or to a different set of receivers connected to the same distribution network. The auxiliary data may be of any general type, including additional encoded video data. Suitable examples of independent libraries of auxiliary data are storage systems containing advertisements, promotional features or previews, news and other informational content, or data for computer-related applications. Such auxiliary data may be either compressed or uncompressed. They may be selectively directed to certain individual receivers or to certain pre-defined subsets of the entire receiver population. Such pre-defined subsets may vary over time in response to certain information that is known, gathered, or inferred by arbitrary means. For example, one subset of receivers may subscribe to a service that provides local news, while a second, possibly overlapping, subset of receivers may choose to receive one of many different categories of advertisements.
Methods for addressing arbitrary subsets of the receiver population are easily implemented within the MPEG-2 standard ISO/IEC 13818 (hereinafter called "MPEG"). Each component of an MPEG-compatible data multiplex is assigned an unique program identification number (PID). For example, a first video program, included in the multiplex, may be comprised of one video stream and two audio streams, and these components may be identified by PID values of 1, 2, and 3 respectively. Similarly, a second video program, included in the same multiplex, may be comprised of one video stream, four audio streams, and one teletext data stream, and these components may be assigned PID values of 4 through 9, respectively. If ten additional streams are to be extracted from an independent library and included in the data multiplex, then these appended streams could be assigned PID values of 10 through 19, respectively. In this way, all audio, video, or data stream components of all programs contained within a multiplex would be assigned unique PID's, regardless of whether the programs originate from an independent library or otherwise.
MPEG's system layer specification also includes provisions for embedding tables or program maps within the bit-stream to identify the different programs contained within the multiplex and to establish a correspondence between these programs and the PIDs associated with their audio, video, or data components. In addition, different types of programs can be classified into groups, and each such group can be assigned a unique identifier. These unique identifiers can be included in the embedded program tables, with one or more identifiers assigned to each program.